I do not mind admitting that I could be wrong; however it offers you two options. Dismiss or Set Reminder. There is the "Find Flights and More" option but even turning this one setting off, Cortana service(s) are still aware of those events; just search for that day of your plans and it will pop up still despite you turning that option off. Feel free to prove otherwise. Edit: Adding some clarification of what we had tested. Feel free to add your notes. Internally, we had somebody do the following. Then forwarded an email to themselves with their pending flight (future plans)... and it was on their calendar. Thinking back, we will have to test the possibility that he did have his calendar synchronized as a possible way this could have appeared. However, on my own personal device and testing device - which is not synchronized with my calendars, if I search for the date for my upcoming travel plans, it shows up. I just have to search for that day.
Turning off tracking for a particular event will stop Cortana from notifying you proactively, but Cortana will keep access to information from your communications. To completely turn off Cortana’s capability to track flights, packages and other information from your messages, go to Cortana > Notebook > Settings, then, under Find flights and more, turn the Detect tracking info, such as flights, in messages on my device setting Off.